The Parallel Seismic method is a borehole seismic technique used to determine the length of foundations (piles, retaining walls, etc) and nearby defects in the ground. Sometimes it is necessary to know the depth of a foundation and it is not possible to obtain this information from direct methods. In this situation, a borehole is drilled as close as possible to the analyzed structure. The borehole is cased (usually with PVC), filled with water and an array of hydrophones (closely spaced every one meter) is introduced inside. Seismic waves are generated hitting the structure at the surface with a hammer. These waves will travel downwards along the structure (with very high velocity) until they reach the ground. Then, they will start travelling much more slowly. The contrast in velocity is observed as a kink in the traveltime-curves of the first arrivals observed in the seismic sections. The point where the velocity decreases rapidly is interpreted as the depth of the structure. The method works best when the contrast is largest (concrete-embedded structures in soils). When the structure is surrounded with sound rock the change is more difficult to identify.
Example of a shot gather from a parallel seismic test to determine the length of a retaining wall